feat(prdy): add PrdService as the single PRD authority surface (#1275)

- PrdService owns create/read/update/link/import/export; wizard and package
  CLI become prompt layers over the service (no second writer path)
- PRD documents gain a content version and a missions linkage array persisted
  in the YAML authority store (survives restart); linkage writes do not bump
  the content version
- exportMarkdown renders a labeled generated view (id + version + do-not-edit
  header) that no code path reads back; the store loads .yaml/.yml only
- importDocument validates structure with zod before anything else, persists
  valid imports as draft (validity is not approval), and refuses conflicts
  with a typed PrdImportConflictError carrying a proposed successor; the
  original document stays byte-identical until acceptSuccessor
- raw store writers (createPrd/savePrd) are no longer exported from the
  package entry point

packages/prdy/src/prd.ts

@@ -17,17 +17,49 @@ const prdSectionSchema = z.object({
   fields: z.record(z.string(), z.string()),
 });
 
+const prdMissionLinkageSchema = z.object({
+  missionId: z.string().min(1),
+  missionVersion: z.string().min(1),
+  prdVersion: z.number().int().min(1),
+  requirementIds: z.array(z.string()),
+  linkedAt: z.string().datetime(),
+});
+
 const prdDocumentSchema = z.object({
   id: z.string().min(1),
   title: z.string().min(1),
   status: z.enum(['draft', 'review', 'approved', 'archived']),
   projectPath: z.string().min(1),
   template: z.string().min(1),
+  // Defaults keep documents written by older prdy versions loadable.
+  version: z.number().int().min(1).default(1),
   sections: z.array(prdSectionSchema),
+  missions: z.array(prdMissionLinkageSchema).default([]),
   createdAt: z.string().datetime(),
   updatedAt: z.string().datetime(),
 });
 
+/** YAML timestamp scalars are parsed as Date by some emitters — normalize to ISO strings. */
+function coerceTimestamps(value: unknown): unknown {
+  if (value instanceof Date) {
+    return value.toISOString();
+  }
+  if (Array.isArray(value)) {
+    return value.map(coerceTimestamps);
+  }
+  if (typeof value === 'object' && value !== null) {
+    return Object.fromEntries(
+      Object.entries(value).map(([key, entry]) => [key, coerceTimestamps(entry)]),
+    );
+  }
+  return value;
+}
+
+/** Validate an unknown value as a PRD document (throws zod errors on failure). */
+export function parsePrdDocument(value: unknown): PrdDocument {
+  return prdDocumentSchema.parse(coerceTimestamps(value)) as PrdDocument;
+}
+
 function expandHome(projectPath: string): string {
   if (!projectPath.startsWith('~')) {
     return projectPath;
@@ -74,6 +106,8 @@ function prdDirectory(projectPath: string): string {
   return path.join(projectPath, PRD_DIRECTORY);
 }
 
+export { prdDirectory };
+
 function prdFilePath(projectPath: string, id: string): string {
   return path.join(prdDirectory(projectPath), `${id}.yaml`);
 }
@@ -113,11 +147,13 @@ export async function createPrd(options: CreatePrdOptions): Promise<PrdDocument>
     status: 'draft',
     projectPath: resolvedProjectPath,
     template: template.id,
+    version: 1,
     sections: template.sections.map((section) => ({
       id: section.id,
       title: section.title,
       fields: Object.fromEntries(section.fields.map((field) => [field, ''])),
     })),
+    missions: [],
     createdAt: now,
     updatedAt: now,
   };
@@ -190,7 +226,7 @@ export async function listPrds(projectPath: string): Promise<PrdDocument[]> {
       throw new Error(`Failed to parse PRD file ${filePath}: ${String(error)}`);
     }
 
-    const document = prdDocumentSchema.parse(parsed);
+    const document = parsePrdDocument(parsed);
     documents.push(document);
   }